Mems device and interconnects for same

ABSTRACT

A microelectromechanical systems device having an electrical interconnect connected to at least one of an electrode and a movable layer within the device. At least a portion of the electrical interconnect is formed from the same material as a movable layer of the device. A thin film, particularly formed of molybdenum, is provided underneath the electrical interconnect. The movable layer preferably comprises aluminum.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/613,922, filed Dec. 20, 2006, the disclosure of which is incorporatedherein by reference.

BACKGROUND

1. Field

The field of the invention relates to microelectromechanical systems(MEMS). More specifically, the field of the invention relates tofabricating electrical interconnects for MEMS.

2. Description of the Related Technology

Microelectromechanical systems (MEMS) include micro mechanical elements,actuators, and electronics. Micromechanical elements may be createdusing deposition, etching, and/or other micromachining processes thatremove parts of substrates and/or deposited material layers or that addlayers to form electrical and electromechanical devices. One type ofMEMS device is called an interferometric modulator. As used herein, theterm interferometric modulator or interferometric light modulator refersto a device that selectively absorbs and/or reflects light using theprinciples of optical interference. In certain embodiments, aninterferometric modulator may comprise a pair of conductive plates, oneor both of which may be transparent and/or reflective in whole or partand capable of relative motion upon application of an appropriateelectrical signal. In a particular embodiment, one plate may comprise astationary layer deposited on a substrate and the other plate maycomprise a metallic membrane separated from the stationary layer by anair gap. As described herein in more detail, the position of one platein relation to another can change the optical interference of lightincident on the interferometric modulator. Such devices have a widerange of applications, and it would be beneficial in the art to utilizeand/or modify the characteristics of these types of devices so thattheir features can be exploited in improving existing products andcreating new products that have not yet been developed.

SUMMARY OF CERTAIN EMBODIMENTS

The system, method, and devices of the invention each have severalaspects, no single one of which is solely responsible for its desirableattributes. Without limiting the scope of this invention, its moreprominent features will now be discussed briefly. After considering thisdiscussion, and particularly after reading the section entitled“Detailed Description of Certain Embodiments” one will understand howthe features of this invention provide advantages over other displaydevices.

An embodiment provides a microelectromechanical systems device includinga lower electrode, a movable conductive layer, a cavity between theelectrode and the movable conductive layer, and an electricalinterconnect connected to at least one of the lower electrode and themovable conductive layer. The electrical interconnect and the movableconductive layer are formed of a same material and the electricalinterconnect layer is formed directly over a thin film that is etchableby fluorine-based etchants.

According to another embodiment, a method is provided for forming amicroelectromechanical systems device. An electrode in an array regionis provided. A sacrificial layer is deposited over the electrode in thearray region. A thin film is deposited over the sacrificial layer and inan interconnect region. A movable layer is formed over the thin film. Anelectrical interconnect layer is over the thin film in the interconnectregion, wherein the electrical interconnect layer comprises a samematerial as the movable layer.

According to yet another embodiment, a microelectromechanical systemsdevice is provided. The device includes a first means for conducting, asecond means for conducting, and a cavity between the first means forconducting and the second means for conducting. The second means forconducting comprises a movable layer. The device also includes aninterconnect means for electrically communicating between circuitryoutside the device and at least one of the first and second means forconducting. The interconnect means and the second means for conductingare formed of a same material and the interconnect means is formeddirectly over a thin film that is etchable by fluorine-based etchants.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be readily apparent fromthe following description and from the appended drawings (not to scale),which are meant to illustrate and not to limit the invention, andwherein:

FIG. 1 is an isometric view depicting a portion of one embodiment of aninterferometric modulator display in which a movable reflective layer ofa first interferometric modulator is in a relaxed position and a movablereflective layer of a second interferometric modulator is in an actuatedposition.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device incorporating a 3×3 interferometric modulator display.

FIG. 3 is a diagram of movable mirror position versus applied voltagefor one exemplary embodiment of an interferometric modulator of FIG. 1.

FIG. 4 is an illustration of a set of row and column voltages that maybe used to drive an interferometric modulator display.

FIGS. 5A and 5B illustrate one exemplary timing diagram for row andcolumn signals that may be used to write a frame of display data to the3×3 interferometric modulator display of FIG. 2.

FIGS. 6A and 6B are system block diagrams illustrating an embodiment ofa visual display device comprising a plurality of interferometricmodulators.

FIG. 7A is a cross section of the device of FIG. 1.

FIG. 7B is a cross section of an alternative embodiment of aninterferometric modulator.

FIG. 7C is a cross section of another alternative embodiment of aninterferometric modulator.

FIG. 7D is a cross section of yet another alternative embodiment of aninterferometric modulator.

FIG. 7E is a cross section of an additional alternative embodiment of aninterferometric modulator.

FIGS. 8A-8N are cross sections showing a process for making anembodiment of an interferometric modulator.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

The following detailed description is directed to certain specificembodiments of the invention. However, the invention can be embodied ina multitude of different ways. In this description, reference is made tothe drawings wherein like parts are designated with like numeralsthroughout. As will be apparent from the following description, theembodiments may be implemented in any device that is configured todisplay an image, whether in motion (e.g., video) or stationary (e.g.,still image), and whether textual or pictorial. More particularly, it iscontemplated that the embodiments may be implemented in or associatedwith a variety of electronic devices such as, but not limited to, mobiletelephones, wireless devices, personal data assistants (PDAs), hand-heldor portable computers, GPS receivers/navigators, cameras, MP3 players,camcorders, game consoles, wrist watches, clocks, calculators,television monitors, flat panel displays, computer monitors, autodisplays (e.g., odometer display, etc.), cockpit controls and/ordisplays, display of camera views (e.g., display of a rear view camerain a vehicle), electronic photographs, electronic billboards or signs,projectors, architectural structures, packaging, and aestheticstructures (e.g., display of images on a piece of jewelry). MEMS devicesof similar structure to those described herein can also be used innon-display applications such as in electronic switching devices.

According to embodiments described herein, a microelectromechanicalsystems (MEMS) device and method for making the device are provided. Thedevice includes an electrical interconnect connected to at least one ofan electrode and a movable layer (e.g., aluminum used as a reflector inan interferometric modulator) within the device. At least a portion ofthe electrical interconnect is formed from the same material as themovable layer of the device. A thin film, preferably a materialsusceptible to the release etch that will remove the sacrificialmaterial from under the movable electrode, is provided under theelectrical interconnect.

One interferometric modulator display embodiment comprising aninterferometric MEMS display element is illustrated in FIG. 1. In thesedevices, the pixels are in either a bright or dark state. In the bright(“on” or “open”) state, the display element reflects a large portion ofincident visible light to a user. When in the dark (“off” or “closed”)state, the display element reflects little incident visible light to theuser. Depending on the embodiment, the light reflectance properties ofthe “on” and “off” states may be reversed. MEMS pixels can be configuredto reflect predominantly at selected colors, allowing for a colordisplay in addition to black and white.

FIG. 1 is an isometric view depicting two adjacent pixels in a series ofpixels of a visual display, wherein each pixel comprises a MEMSinterferometric modulator. In some embodiments, an interferometricmodulator display comprises a row/column array of these interferometricmodulators. Each interferometric modulator includes a pair of reflectivelayers positioned at a variable and controllable distance from eachother to form a resonant optical gap with at least one variabledimension. In one embodiment, one of the reflective layers may be movedbetween two positions. In the first position, referred to herein as therelaxed position, the movable reflective layer is positioned at arelatively large distance from a fixed partially reflective layer. Inthe second position, referred to herein as the actuated position, themovable reflective layer is positioned more closely adjacent to thepartially reflective layer. Incident light that reflects from the twolayers interferes constructively or destructively depending on theposition of the movable reflective layer, producing either an overallreflective or non-reflective state for each pixel.

The depicted portion of the pixel array in FIG. 1 includes two adjacentinterferometric modulators 12 a and 12 b. In the interferometricmodulator 12 a on the left, a movable reflective layer 14 a isillustrated in a relaxed position at a predetermined distance from anoptical stack 16 a, which includes a partially reflective layer. In theinterferometric modulator 12 b on the right, the movable reflectivelayer 14 b is illustrated in an actuated position adjacent to theoptical stack 16 b.

The optical stacks 16 a and 16 b (collectively referred to as opticalstack 16), as referenced herein, typically comprise several fusedlayers, which can include an electrode layer, such as indium tin oxide(ITO), a partially reflective layer, such as chromium, and a transparentdielectric. The optical stack 16 is thus electrically conductive,partially transparent, and partially reflective, and may be fabricated,for example, by depositing one or more of the above layers onto atransparent substrate 20. The partially reflective layer can be formedfrom a variety of materials that are partially reflective such asvarious metals, semiconductors, and dielectrics. The partiallyreflective layer can be formed of one or more layers of materials, andeach of the layers can be formed of a single material or a combinationof materials.

In some embodiments, the layers of the optical stack 16 are patternedinto parallel strips, and may form row electrodes in a display device asdescribed further below. The movable reflective layers 14 a, 14 b may beformed as a series of parallel strips of a deposited metal layer orlayers (orthogonal to the row electrodes of 16 a, 16 b) deposited on topof posts 18 and an intervening sacrificial material deposited betweenthe posts 18. When the sacrificial material is etched away, the movablereflective layers 14 a, 14 b are separated from the optical stacks 16 a,16 b by a defined gap 19. A highly conductive and reflective materialsuch as aluminum may be used for the reflective layers 14, and thesestrips may form column electrodes in a display device.

With no applied voltage, the gap 19 remains between the movablereflective layer 14 a and optical stack 16 a, with the movablereflective layer 14 a in a mechanically relaxed state, as illustrated bythe pixel 12 a in FIG. 1. However, when a potential difference isapplied to a selected row and column, the capacitor formed at theintersection of the row and column electrodes at the corresponding pixelbecomes charged, and electrostatic forces pull the electrodes together.If the voltage is high enough, the movable reflective layer 14 isdeformed and is forced against the optical stack 16. A dielectric layer(not illustrated in this Figure) within the optical stack 16 may preventshorting and control the separation distance between layers 14 and 16,as illustrated by pixel 12 b on the right in FIG. 1. The behavior is thesame regardless of the polarity of the applied potential difference. Inthis way, row/column actuation that can control the reflective vs.non-reflective pixel states is analogous in many ways to that used inconventional LCD and other display technologies.

FIGS. 2 through 5B illustrate one exemplary process and system for usingan array of interferometric modulators in a display application.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device that may incorporate aspects of the invention. In theexemplary embodiment, the electronic device includes a processor 21which may be any general purpose single- or multi-chip microprocessorsuch as an ARM, Pentium®, Pentium II®, Pentium III®, Pentium IV®,Pentium® Pro, an 8051, a MIPS®, a Power PC®, an ALPHA®, or any specialpurpose microprocessor such as a digital signal processor,microcontroller, or a programmable gate array. As is conventional in theart, the processor 21 may be configured to execute one or more softwaremodules. In addition to executing an operating system, the processor maybe configured to execute one or more software applications, including aweb browser, a telephone application, an email program, or any othersoftware application.

In one embodiment, the processor 21 is also configured to communicatewith an array driver 22. In one embodiment, the array driver 22 includesa row driver circuit 24 and a column driver circuit 26 that providesignals to a display array or panel 30. The cross section of the arrayillustrated in FIG. 1 is shown by the lines 1-1 in FIG. 2. For MEMSinterferometric modulators, the row/column actuation protocol may takeadvantage of a hysteresis property of these devices illustrated in FIG.3. It may require, for example, a 10 volt potential difference to causea movable layer to deform from the relaxed state to the actuated state.However, when the voltage is reduced from that value, the movable layermaintains its state as the voltage drops back below 10 volts. In theexemplary embodiment of FIG. 3, the movable layer does not relaxcompletely until the voltage drops below 2 volts. Thus, there exists awindow of applied voltage, about 3 to 7 V in the example illustrated inFIG. 3, within which the device is stable in either the relaxed oractuated state. This is referred to herein as the “hysteresis window” or“stability window.” For a display array having the hysteresischaracteristics of FIG. 3, the row/column actuation protocol can bedesigned such that during row strobing, pixels in the strobed row thatare to be actuated are exposed to a voltage difference of about 10volts, and pixels that are to be relaxed are exposed to a voltagedifference of close to zero volts. After the strobe, the pixels areexposed to a steady state voltage difference of about 5 volts such thatthey remain in whatever state the row strobe put them in. After beingwritten, each pixel sees a potential difference within the “stabilitywindow” of 3-7 volts in this example. This feature makes the pixeldesign illustrated in FIG. 1 stable under the same applied voltageconditions in either an actuated or relaxed pre-existing state. Sinceeach pixel of the interferometric modulator, whether in the actuated orrelaxed state, is essentially a capacitor formed by the fixed and movingreflective layers, this stable state can be held at a voltage within thehysteresis window with almost no power dissipation. Essentially nocurrent flows into the pixel if the applied potential is fixed.

In typical applications, a display frame may be created by asserting theset of column electrodes in accordance with the desired set of actuatedpixels in the first row. A row pulse is then applied to the row 1electrode, actuating the pixels corresponding to the asserted columnlines. The asserted set of column electrodes is then changed tocorrespond to the desired set of actuated pixels in the second row. Apulse is then applied to the row 2 electrode, actuating the appropriatepixels in row 2 in accordance with the asserted column electrodes. Therow 1 pixels are unaffected by the row 2 pulse, and remain in the statethey were set to during the row 1 pulse. This may be repeated for theentire series of rows in a sequential fashion to produce the frame.Generally, the frames are refreshed and/or updated with new display databy continually repeating this process at some desired number of framesper second. A wide variety of protocols for driving row and columnelectrodes of pixel arrays to produce display frames are also well knownand may be used in conjunction with the present invention.

FIGS. 4, 5A, and 5B illustrate one possible actuation protocol forcreating a display frame on the 3×3 array of FIG. 2. FIG. 4 illustratesa possible set of column and row voltage levels that may be used forpixels exhibiting the hysteresis curves of FIG. 3. In the FIG. 4embodiment, actuating a pixel involves setting the appropriate column to−V_(bias), and the appropriate row to +ΔV, which may correspond to −5volts and +5 volts, respectively Relaxing the pixel is accomplished bysetting the appropriate column to +V_(bias), and the appropriate row tothe same +ΔV, producing a zero volt potential difference across thepixel. In those rows where the row voltage is held at zero volts, thepixels are stable in whatever state they were originally in, regardlessof whether the column is at +V_(bias), or −V_(bias). As is alsoillustrated in FIG. 4, it will be appreciated that voltages of oppositepolarity than those described above can be used, e.g., actuating a pixelcan involve setting the appropriate column to +V_(bias), and theappropriate row to −ΔV. In this embodiment, releasing the pixel isaccomplished by setting the appropriate column to −V_(bias), and theappropriate row to the same −ΔV, producing a zero volt potentialdifference across the pixel.

FIG. 5B is a timing diagram showing a series of row and column signalsapplied to the 3×3 array of FIG. 2 which will result in the displayarrangement illustrated in FIG. 5A, where actuated pixels arenon-reflective. Prior to writing the frame illustrated in FIG. 5A, thepixels can be in any state, and in this example, all the rows are at 0volts, and all the columns are at +5 volts. With these applied voltages,all pixels are stable in their existing actuated or relaxed states.

In the FIG. 5A frame, pixels (1,1), (1,2), (2,2), (3,2) and (3,3) areactuated. To accomplish this, during a “line time” for row 1, columns 1and 2 are set to −5 volts, and column 3 is set to +5 volts. This doesnot change the state of any pixels, because all the pixels remain in the3-7 volt stability window. Row 1 is then strobed with a pulse that goesfrom 0, up to 5 volts, and back to zero. This actuates the (1,1) and(1,2) pixels and relaxes the (1,3) pixel. No other pixels in the arrayare affected. To set row 2 as desired, column 2 is set to −5 volts, andcolumns 1 and 3 are set to +5 volts. The same strobe applied to row 2will then actuate pixel (2,2) and relax pixels (2,1) and (2,3). Again,no other pixels of the array are affected. Row 3 is similarly set bysetting columns 2 and 3 to −5 volts, and column 1 to +5 volts. The row 3strobe sets the row 3 pixels as shown in FIG. 5A. After writing theframe, the row potentials are zero, and the column potentials can remainat either +5 or −5 volts, and the display is then stable in thearrangement of FIG. 5A. It will be appreciated that the same procedurecan be employed for arrays of dozens or hundreds of rows and columns. Itwill also be appreciated that the timing, sequence, and levels ofvoltages used to perform row and column actuation can be varied widelywithin the general principles outlined above, and the above example isexemplary only, and any actuation voltage method can be used with thesystems and methods described herein.

FIGS. 6A and 6B are system block diagrams illustrating an embodiment ofa display device 40. The display device 40 can be, for example, acellular or mobile telephone. However, the same components of displaydevice 40 or slight variations thereof are also illustrative of varioustypes of display devices such as televisions and portable media players.

The display device 40 includes a housing 41, a display 30, an antenna43, a speaker 45, an input device 48, and a microphone 46. The housing41 is generally formed from any of a variety of manufacturing processesas are well known to those of skill in the art, including injectionmolding and vacuum forming. In addition, the housing 41 may be made fromany of a variety of materials, including, but not limited to, plastic,metal, glass, rubber, and ceramic, or a combination thereof. In oneembodiment, the housing 41 includes removable portions (not shown) thatmay be interchanged with other removable portions of different color, orcontaining different logos, pictures, or symbols.

The display 30 of the exemplary display device 40 may be any of avariety of displays, including a bi-stable display, as described herein.In other embodiments, the display 30 includes a flat-panel display, suchas plasma, EL, OLED, STN LCD, or TFT LCD as described above, or anon-flat-panel display, such as a CRT or other tube device, as is wellknown to those of skill in the art. However, for purposes of describingthe present embodiment, the display 30 includes an interferometricmodulator display, as described herein.

The components of one embodiment of exemplary display device 40 areschematically illustrated in FIG. 6B. The illustrated exemplary displaydevice 40 includes a housing 41 and can include additional components atleast partially enclosed therein. For example, in one embodiment, theexemplary display device 40 includes a network interface 27 thatincludes an antenna 43, which is coupled to a transceiver 47. Thetransceiver 47 is connected to a processor 21, which is connected toconditioning hardware 52. The conditioning hardware 52 may be configuredto condition a signal (e.g., filter a signal). The conditioning hardware52 is connected to a speaker 45 and a microphone 46. The processor 21 isalso connected to an input device 48 and a driver controller 29. Thedriver controller 29 is coupled to a frame buffer 28 and to an arraydriver 22, which in turn is coupled to a display array 30. A powersupply 50 provides power to all components as required by the particularexemplary display device 40 design.

The network interface 27 includes the antenna 43 and the transceiver 47so that the exemplary display device 40 can communicate with one or moredevices over a network. In one embodiment, the network interface 27 mayalso have some processing capabilities to relieve requirements of theprocessor 21. The antenna 43 is any antenna known to those of skill inthe art for transmitting and receiving signals. In one embodiment, theantenna transmits and receives RF signals according to the IEEE 802.11standard, including IEEE 802.11(a), (b), or (g). In another embodiment,the antenna transmits and receives RF signals according to the BLUETOOTHstandard. In the case of a cellular telephone, the antenna is designedto receive CDMA, GSM, AMPS, or other known signals that are used tocommunicate within a wireless cell phone network. The transceiver 47pre-processes the signals received from the antenna 43 so that they maybe received by and further manipulated by the processor 21. Thetransceiver 47 also processes signals received from the processor 21 sothat they may be transmitted from the exemplary display device 40 viathe antenna 43.

In an alternative embodiment, the transceiver 47 can be replaced by areceiver. In yet another alternative embodiment, the network interface27 can be replaced by an image source, which can store or generate imagedata to be sent to the processor 21. For example, the image source canbe a digital video disc (DVD) or a hard-disc drive that contains imagedata, or a software module that generates image data.

The processor 21 generally controls the overall operation of theexemplary display device 40. The processor 21 receives data, such ascompressed image data from the network interface 27 or an image source,and processes the data into raw image data or into a format that isreadily processed into raw image data. The processor 21 then sends theprocessed data to the driver controller 29 or to the frame buffer 28 forstorage. Raw data typically refers to the information that identifiesthe image characteristics at each location within an image. For example,such image characteristics can include color, saturation, and gray-scalelevel.

In one embodiment, the processor 21 includes a microcontroller, CPU, orlogic unit to control operation of the exemplary display device 40. Theconditioning hardware 52 generally includes amplifiers and filters fortransmitting signals to the speaker 45, and for receiving signals fromthe microphone 46. The conditioning hardware 52 may be discretecomponents within the exemplary display device 40, or may beincorporated within the processor 21 or other components.

The driver controller 29 takes the raw image data generated by theprocessor 21 either directly from the processor 21 or from the framebuffer 28 and reformats the raw image data appropriately for high speedtransmission to the array driver 22. Specifically, the driver controller29 reformats the raw image data into a data flow having a raster-likeformat, such that it has a time order suitable for scanning across thedisplay array 30. Then the driver controller 29 sends the formattedinformation to the array driver 22. Although a driver controller 29,such as a LCD controller, is often associated with the system processor21 as a stand-alone Integrated Circuit (IC), such controllers may beimplemented in many ways. They may be embedded in the processor 21 ashardware, embedded in the processor 21 as software, or fully integratedin hardware with the array driver 22.

Typically, the array driver 22 receives the formatted information fromthe driver controller 29 and reformats the video data into a parallelset of waveforms that are applied many times per second to the hundredsand sometimes thousands of leads coming from the display's x-y matrix ofpixels.

In one embodiment, the driver controller 29, array driver 22, anddisplay array 30 are appropriate for any of the types of displaysdescribed herein. For example, in one embodiment, the driver controller29 is a conventional display controller or a bi-stable displaycontroller (e.g., an interferometric modulator controller). In anotherembodiment, the array driver 22 is a conventional driver or a bi-stabledisplay driver (e.g., an interferometric modulator display). In oneembodiment, the driver controller 29 is integrated with the array driver22. Such an embodiment is common in highly integrated systems such ascellular phones, watches, and other small area displays. In yet anotherembodiment, the display array 30 is a typical display array or abi-stable display array (e.g., a display including an array ofinterferometric modulators).

The input device 48 allows a user to control the operation of theexemplary display device 40. In one embodiment, the input device 48includes a keypad, such as a QWERTY keyboard or a telephone keypad, abutton, a switch, a touch-sensitive screen, or a pressure- orheat-sensitive membrane. In one embodiment, the microphone 46 is aninput device for the exemplary display device 40. When the microphone 46is used to input data to the device, voice commands may be provided by auser for controlling operations of the exemplary display device 40.

The power supply 50 can include a variety of energy storage devices asare well known in the art. For example, in one embodiment, the powersupply 50 is a rechargeable battery, such as a nickel-cadmium battery ora lithium ion battery. In another embodiment, the power supply 50 is arenewable energy source, a capacitor, or a solar cell including aplastic solar cell, and solar-cell paint. In another embodiment, thepower supply 50 is configured to receive power from a wall outlet.

In some embodiments, control programmability resides, as describedabove, in a driver controller which can be located in several places inthe electronic display system. In some embodiments, controlprogrammability resides in the array driver 22. Those of skill in theart will recognize that the above-described optimizations may beimplemented in any number of hardware and/or software components and invarious configurations.

The details of the structure of interferometric modulators that operatein accordance with the principles set forth above may vary widely. Forexample, FIGS. 7A-7E illustrate five different embodiments of themovable reflective layer 14 and its supporting structures. FIG. 7A is across section of the embodiment of FIG. 1, where a strip of metalmaterial 14 is deposited on orthogonally extending supports 18. Thesupports 18 can comprise isolated posts or continuous walls. Forexample, the supports 18 can include linear rails that support crossingstrips of mechanical or movable material, and/or isolated posts. In oneexample, rails provide primary support and posts within each cavityserve to stiffen the mechanical layer.

In FIG. 7B, the moveable reflective layer 14 is attached to supports atthe corners only, on tethers 32. In FIG. 7C, the moveable reflectivelayer 14 is suspended from a deformable mechanical layer 34, which maycomprise a flexible metal. The deformable mechanical layer 34 connects,directly or indirectly, to the substrate 20 around the perimeter of thedeformable mechanical layer 34. These connections are herein referred toas support structures or supports 18. The embodiment illustrated in FIG.7D has supports 18 that include post plugs 42 upon which the deformablelayer 34 rests. The movable reflective layer 14 remains suspended overthe gap, as in FIGS. 7A-7C, but the mechanical layer 34 does not formthe support posts by filling holes between the mechanical layer 34 andthe optical stack 16. Rather, supports 18 are separately deposited underthe mechanical layer 34. The embodiment illustrated in FIG. 7E is basedon the embodiment shown in FIG. 7D, but may also be adapted to work withany of the embodiments illustrated in FIGS. 7A-7C, as well as additionalembodiments not shown. In the embodiment shown in FIG. 7E, an extralayer of metal or other conductive material has been used to form a busstructure 44. This allows signal routing along the back of theinterferometric modulators, eliminating a number of electrodes that mayotherwise have had to be formed on the substrate 20.

In embodiments such as those shown in FIG. 7A-7E, the interferometricmodulators function as direct-view devices, in which images are viewedfrom the front side of the transparent substrate 20, the side oppositeto that upon which the modulator is arranged. In these embodiments, thereflective layer 14 optically shields the portions of theinterferometric modulator on the side of the reflective layer oppositethe substrate 20, including the deformable layer 34. This allows theshielded areas to be configured and operated upon without negativelyaffecting the image quality. Such shielding allows the bus structure 44in FIG. 7E, which provides the ability to separate the opticalproperties of the modulator from the electromechanical properties of themodulator, such as addressing and the movements that result from thataddressing. This separable modulator architecture allows the structuraldesign and materials used for the electromechanical aspects and theoptical aspects of the modulator to be selected and to functionindependently of each other. Moreover, the embodiments shown in FIGS.7C-7E have additional benefits deriving from the decoupling of theoptical properties of the reflective layer 14 from its mechanicalproperties, which are carried out by the deformable mechanical layer 34.This allows the structural design and materials used for the reflectivelayer 14 to be optimized with respect to the optical properties, and thestructural design and materials used for the deformable layer 34 to beoptimized with respect to desired mechanical properties.

Layers, materials, and/or other structural elements may be describedherein as being “over,” “above,” “between,” etc. in relation to otherstructural elements. As used herein, these terms can mean directly orindirectly on, over, above, between, etc., as a variety of intermediatelayers, material, and/or other structural elements can be interposedbetween structural elements described herein. Similarly, structuralelements described herein, such as substrates or layers, can comprise asingle component (e.g., a monolayer) or a multi-component structure(e.g., a laminate comprising multiple layers of the recited material,with or without layers of additional materials). Use of the term “one ormore” with respect to an object or element does not, in any way,indicate the absence of a potential plural arrangement of objects orelements for which the term is not used. The term“microelectromechanical device,” as used herein, refers generally to anysuch device at any stage of manufacture.

Methods disclosed herein employ depositions of conductive layers for usein the MEMS array to simultaneously form peripheral electricalinterconnect or routing. In some options for forming amicroelectromechanical system (e.g., an interferometric modulator),depositions that form the deformable mechanical layer 34 and/or theelectrodes of the optical stack 16, can also be used to provideelectrical interconnect and routing in the periphery of the display,where the interconnect is between circuitry outside the array (e.g.,driver chip(s) at a contact pad) and an electrode (row or column) withinthe array.

As discussed above, certain embodiments of MEMS devices, and inparticular interferometric modulators, comprise a movable layercomprising a reflective layer that is partially detached from amechanical or deformable layer (e.g., FIGS. 7C-7E). An exemplary processwill be described with reference to FIGS. 8A-8N. It will be understoodthat FIGS. 8A-8N are cross-sectional views of the row electrodes of thedevice. According to this embodiment, the peripheralrouting/interconnect is formed from the same material that is used toform the reflective layer (e.g., aluminum mirror) in the movable layerwithin the MEMS device, and preferably formed from the same deposition.

With reference to FIG. 8A, an optical stack 16 is formed over thetransparent substrate 20 in one embodiment. The ITO 16 a of the opticalstack 16 may be deposited by standard deposition techniques, includingchemical vapor deposition (CVD) or sputtering. A relatively thinabsorber layer 16 b of, e.g., MoCr or Cr, is preferably deposited overthe ITO 16 a. The ITO 16 a and MoCr or Cr 16 b are then etched andpatterned into rows to form the electrodes of the optical stack 16. Asdescribed above, the optical stack 16 includes a dielectric layer 16 c(e.g., silicon dioxide (SiO₂)) to provide electrical isolation duringoperation between the row electrodes and subsequently deposited columnelectrodes. The dielectric layer 16 c can be deposited before or afterpatterning the row electrodes. The silicon dioxide 16 c may be coveredwith an aluminum oxide (Al₂O₃) cap layer 16 d to protect it from therelease etch performed later in the fabrication sequence. In somearrangements, a further etch stop is formed over the Al₂O₃ layer 16 d toprotect it during subsequent patterning steps to define multiplethicknesses of sacrificial material to define multiple cavity sizes andcorresponding colors.

As shown in FIG. 8B, a sacrificial layer (or layers) 82, preferablycomprising a material that can be selectively etched by fluorine-basedetchants, and particularly molybdenum (Mo), is deposited (and laterpartially removed in the release etch) over the structure. Thesacrificial layer 82 preferably comprises a metal that is selectivelyetchable, relative to the dielectric of the optical stack 16 and otherexposed metals of the MEMS device. In certain embodiments, thissacrificial layer 82 may comprise, for example, tungsten (W), titanium(Ti), or amorphous silicon.

As illustrated in FIG. 8C, according to this embodiment, the sacrificiallayer 82 is patterned such that it remains only in the image (or “array”or “display”) area. It will be understood that the sacrificial material82 is preferably deposited (and later selectively removed) over theoptical stack 16 to define a resonant optical cavity 19 (FIG. 8M)between the optical stack 16 and a movable layer that will be deposited,as described in more detail below. It will be understood that thesacrificial layer 82 may comprise multiple layers that are deposited andsubsequently patterned to form a sacrificial layer 82 having multiplethicknesses to produce interferometric modulators for reflectingmultiple different colors, such as red, green, and blue for an RGBdisplay system. As shown in FIGS. 8B and 8C, the sacrificial layer 82has varying thicknesses. The skilled artisan will understand that thesevarying thicknesses correspond to varying heights of the cavity 19 (FIG.8M) that is formed when the sacrificial layer 82 is removed, as will bedescribed below. In the process of forming the three different heights,the thinnest of the three illustrated portions of the sacrificial layercan be formed from a single deposited layer; the intermediate thicknessformed from two depositions; and the thickest portion formed from threedepositions. Etch stop layers can optionally intervene between thedepositions. In an exemplary embodiment, a modulator having a cavitywith the largest height (formed by a sacrificial layer having thegreatest thickness) reflects red light, a modulator having a cavity withan intermediate height (formed by a sacrificial layer having anintermediate thickness) reflects green light, and a modulator having acavity with the smallest height (formed by a sacrificial layer havingthe smallest thickness) reflects blue light. For simplicity, thesacrificial layer 82 is shown as a single layer in the figures.

In this embodiment, as shown in FIG. 8D, a thin film 64 is depositedover the entire structure, including the interconnect region. The thinfilm 64 is preferably selectively etchable by the same etchant thatremoves the sacrificial layer, and is more preferably formed of the samematerial as the sacrificial layer 82. In a preferred embodiment, boththe thin film 64 and the sacrificial layer 82 are formed of molybdenum.Preferably, the thin film 64 has a thickness in the range of about 50Å-500 Å, more preferably in the range of about 80 Å-200 Å, and even morepreferably is about 100 Å thick. As will described below, this thin film64 can also function as a barrier layer in the interconnect region.Although the thin film 64 is illustrated as being deposited over theentire structure, as shown in FIG. 8D, it will be understood that, inalternative embodiments, the thin film 64 is deposited only in theinterconnect region. Thus, it will be understood that the thin film 64is deposited at least in the interconnect region.

As shown in FIG. 8E, a conductive layer 90, preferably a reflectivelayer, is deposited over the thin film 64 to form the movable electrodes14 of the interferometric modulator and at least a portion of theelectrical interconnect or routing 202 (FIG. 8F). As will be describedin more detail below, the interconnect or routing 202 electricallyconnects circuitry on a contact pad outside the array with either therow electrode (e.g., ITO of the optical stack 16) or the patternedelectrode 14 or both.

For the preferred interferometric modulator embodiment, the conductivelayer 90 is preferably formed of a specular metal material, such asaluminum or an aluminum alloy, such that it will be referred to hereinas a reflective layer, and the patterned movable electrode will bereferred to as the patterned mirror 14. According to certainembodiments, the reflective layer 90 comprises a single layer ofreflective material. In other embodiments, the reflective layer 90 maycomprise a thin layer of reflective material with a layer of more rigidmaterial (not shown) overlying the thin layer of reflective material. Asthe reflective layer of this embodiment will be partially detached froman overlying mechanical layer (FIGS. 8M and 8N) in the image area, thereflective layer 90 preferably has sufficient rigidity to remain in asubstantially flat position relative to the optical stack 16 even whenpartially detached, and the inclusion of a stiffening layer on the sideof the reflective layer located away from the optical stack can be usedto provide the desired rigidity.

After the reflective layer 90 is deposited, it is patterned and etched,as shown in FIG. 8F. The reflective layer 90 is patterned into columnsorthogonal to the row electrodes of the optical stack 16 to create therow/column array described above. These columns form the patternedmirror layer 14 in the image area. According to an embodiment, thereflective layer 90 is patterned at the same time in the peripheralareas to form the electrical interconnect or routing 202 in theperiphery of the display, where the interconnect is between circuitryoutside the array and an electrode within the array. The skilled artisanwill appreciate that in some embodiments, the reflective layer 90 ispatterned simultaneously to form both the electricalinterconnect/routing 202 and the patterned mirror layer 14. In otherembodiments, the patterned mirror layer 14 and the electricalinterconnect/routing 202 may be patterned in separate steps. It willalso be understood that although, the electrical interconnect/routing202 and the patterned mirror layer 14 are formed from a single depositedlayer 90 in the illustrated embodiment, the electricalinterconnect/routing and the patterned mirror layer may be formed fromseparately deposited layers in other embodiments. It will be appreciatedthat the electrical interconnect/routing 202 and the patterned mirrorlayer 14 are formed of the same material.

Typically, a wet etching process (e.g., using phosphoric acid) is usedto pattern the reflective layer 90 to form the patterned mirror 14. Asdescribed above, a cap layer of Al₂O₃ can be deposited over thedielectric layer of the optical stack 16. During such a wet etchingprocess, the wet etchant (e.g., phosphoric acid) can also etch the Al₂O₃cap layer. The etching of the Al₂O₃ exposes the silicon dioxidedielectric underneath in the interconnect region. Typically, during thewet etching process used to pattern the reflective layer 90, the etchrate of the layer 90 in the image or display area is faster than theetch rate of the layer 90 in the interconnect region due to the galvaniceffect caused by the conductive sacrificial layer 82 underneath thereflective layer in the image or display area. As the etch rate of thereflective layer 90 in the interconnect region is slower than the etchrate of the reflective layer 90 in the image area, complete etching(without overetch) of the reflective layer 90 in the image area wouldresult in “incomplete” patterning of the reflective layer 90 in theinterconnect area to form the interconnect/routing 202. The skilledartisan will readily appreciate that such “incomplete” etching leads toshorting. Similarly, if the reflective layer 90 is etched “completely”to form the interconnect/routing, the galvanic effect causes overetchingof the reflective layer 90 in the image area.

According to this embodiment, the thin film 64 also functions as an etchstop layer to protect the Al₂O₃ of the optical stack 16 in theinterconnect region during etching of the reflective layer 90. As thethin film 64 preferably comprises a metal, the thin film 64 alsoprovides the galvanic effect in the interconnect region. The galvaniceffect in both the interconnect region and the image area results inuniform etch rates of the reflective layer 90 in both the interconnectregion and the image area. Thus, according to this embodiment, thereflective layer 90 is etched completely to form theinterconnect/routing 202 and the patterned mirror layer 14. It will beunderstood that the thin film 64 is not necessary in the image ordisplay area to provide the galvanic effect because the sacrificiallayer 82, which is formed of a metal, can provide the galvanic effect.It will be understood that, in an alternative embodiment, if the thinfilm 64 comprises amorphous silicon and the sacrificial layer 82comprises a metal, the thin film 64 is deposited in both the image ordisplay area as well as the interconnect region to negate the galvaniceffect provided by the metal sacrificial layer 82.

As illustrated in FIG. 8G, a second sacrificial layer 196 is depositedover the entire structure, over both the patterned mirror layer 14 inthe image area as well as over the interconnect/routing 202. Whilereferred to herein as a “second” sacrificial layer 196, it will beunderstood that it may in fact represent the fourth deposition ofsacrificial material, due to the use of three depositions to define thethree cavity heights. Preferably, the second sacrificial layer 196 isformed from the same material as the first sacrificial layer 82, or,alternatively, is etchable selectively with respect to the surroundingmaterials by the same etchant (preferably a fluorine-based etchant) asthe first sacrificial layer 82. As illustrated in FIG. 8H, the secondsacrificial layer 196 and the thin film 64 are patterned and taperedapertures 86 are formed and extend through the second sacrificial layer196, the first sacrificial layer 82, and the thin film 64, therebypatterning the sacrificial layers 196, 82 and the thin film 64.

As shown in FIG. 8I, a post material 210 is deposited over the entirestructure. It will be understood that the post material 210 comprises anorganic or inorganic material, but preferably comprises an inorganicmaterial (e.g., oxide, particularly SiO₂). As illustrated in FIG. 8J,this post oxide material 210 is then patterned to form supports 18 forthe device in the image or display area. It can also be seen in FIG. 8Jthat apertures 208 are formed in portions of the second sacrificiallayer 196 overlying the patterned mirror layer 14, exposing at least aportion of the patterned mirror layer 14. In the interconnect region,the post material 210 is patterned to expose portions of theinterconnect 202 to form contacts, as will be described below. Theskilled artisan will appreciate that the post material 210 can also bepatterned to passivate the interconnect/routing 202.

As shown in FIG. 8K, a mechanical layer 92 is deposited over thepatterned post material 210 and exposed portions of the patterned mirrorlayer 14. According to a preferred embodiment, the mechanical layer 92is formed of nickel. In particular, it can be seen that the mechanicallayer 92 at least partially fills the aperture 208 (FIG. 8I) such that aconnector portion 204 connecting the mechanical layer 92 and thepatterned mirror layer 14 is formed. As illustrated in FIG. 8K, themechanical layer 92 contacts the interconnects 202 in the interconnectregion in the exposed areas where the post material 210 is removed (FIG.8J). As shown in FIG. 8L, the mechanical layer 92 is then patterned toform the column electrodes described above. The skilled artisan willappreciate that small holes (not shown) are preferably also etched inthe mechanical layer 92 to aid removal of the sacrificial layers 82, 196by a release etch, using a release etchant, as described below.

As discussed above, an interferometric modulator is configured toreflect light through the transparent substrate and includes movingparts, such as the movable patterned mirror 14. Therefore, to allow suchmoving parts to move, a gap or cavity 19 (as illustrated in FIG. 8M) ispreferably created between the electrodes of the optical stack 16 andthe patterned mirror 14 by selectively removing the sacrificial layers82, 196 (and the thin film 64) in the image (or “array” or “display”)area to create the cavity 19 (FIG. 8M), which will be described in moredetail below. The gap or cavity 19 allows the mechanical parts, such asthe movable mirror 14, of the interferometric modulator to move.

As illustrated in FIG. 8M, after the mechanical layer 92 is formed, arelease etch is performed to remove both the first sacrificial layer 82and the second sacrificial layer 196 as well as the thin film 64 in theimage area, thereby forming an optical gap or cavity 19 between thepatterned mirror layer 14 and the optical stack 16. Thus, an opticalMEMS device is formed, which includes a mechanical layer 92 from which apatterned mirror layer 14 is suspended, where the patterned mirror layer14 is partially detached from the mechanical layer 92. Standard releasetechniques may be used to remove the sacrificial layers 82, 196 and thethin film 64 in the image or display area. The particular releasetechnique will depend on the material to be removed. For example,fluorine-based etchants, and preferably xenon difluoride (XeF₂), may beused to remove molybdenum (illustrated), tungsten, or titanium to createthe cavity 19. It can be seen in FIG. 8M that the post oxide material210 and the mechanical layer material 92 protects theinterconnect/routing 202 from the release etch, including the underlyingremaining thin film 64, which would be otherwise susceptible to therelease etch.

The illustrated MEMS device thus includes an electrical interconnect orrouting 202, which electrically connects circuitry outside the arraywith the electrodes 14, 16 within the array. It will be understood thatthe electrical connectivity path between the electricalinterconnect/routing 202 and the mirror layer electrode 14 is throughthe mechanical layer 92 and the electrical connectivity path between theelectrical interconnect/routing 202 and the ITO 16 a of the opticalstack 16 is through patterned apertures through the layers 16 b-d of theoptical stack 16. The interconnect/routing 202 is formed by patterningthe reflective layer 90, which preferably comprises aluminum. It will beunderstood that, in this embodiment, the same material used to form thepatterned mirror layer 14 is used to form the electricalinterconnect/routing 202.

The skilled artisan will appreciate aluminum (of the movable layer 14)forms a poor and unreliable electrical contact with the ITO (of theelectrodes of the optical stack 16), and that this electrical contactstructure can be made stable and reliable by adding a barrier formed ofa refractory metal (e.g., molybdenum, tungsten, or titanium, or asuitable alloy), between the aluminum and ITO. The illustrated thin film64 (preferably comprising molybdenum) remaining in the interconnectregion between the movable layer 14 and the optical stack 16 improvesboth the contact resistance and reliability. The skilled artisan willalso appreciate that molybdenum may also be used for electrical contactsacross scribe edges and is sufficiently resistant to corrosion that itmay be left exposed at the edges.

After the release etch, a backplate 80 is preferably sealed to thetransparent substrate 20 using a seal 98 to further protect the displayarea of the device. As shown in FIG. 8N, the seal 98 is in the area ofthe peripheral routing/interconnect 202. The movable layer 14 extendsunder the seal 98 and leads to contact pads in the “pad” region whererow/column drivers are mounted. It will be understood that, as notedabove, the figures are not drawn to scale.

The backplate 80 protects the interferometric modulator from harmfulelements in the environment. Similarly, the seal 98 is preferably ahermetic seal for preventing water vapor and other contaminants fromentering the package and damaging the interferometric modulator. Theskilled artisan will understand that transparent substrate 20 may be anytransparent substance capable of having thin film, MEMS devices builtupon it. Such transparent substances include, but are not limited to,glass, plastic, and transparent polymers. Images are displayed throughthe transparent substrate 20.

It will be understood that, in the embodiments described herein, areflective material is deposited between the electrode of the opticalstack and the mechanical layer to form an upper electrode and the samelayer of reflective material is also used for an electrical interconnector routing. This optical MEMS device may be, for example, aninterferometric modulator such as that described with respect to FIGS.7C-7E and elsewhere throughout the application. The skilled artisan willunderstand that in non-optical MEMS devices, the suspended upperelectrode need not be reflective.

While the above detailed description has shown, described, and pointedout novel features of the invention as applied to various embodiments,it will be understood that various omissions, substitutions, and changesin the form and details of the device or process illustrated may be madeby those skilled in the art without departing from the spirit of theinvention. As will be recognized, the present invention may be embodiedwithin a form that does not provide all of the features and benefits setforth herein, as some features may be used or practiced separately fromothers.

1. A method of forming a microelectromechanical systems device,comprising: providing an electrode in an array region; depositing asacrificial layer over the electrode in the array region; depositing athin film over the sacrificial layer and in an interconnect region;forming a movable layer over the thin film; and forming an electricalinterconnect layer over the thin film in the interconnect region,wherein the electrical interconnect layer comprises a same material asthe movable layer.
 2. The method of claim 1, further comprising forminga mechanical layer over the movable layer.
 3. The method of claim 1,wherein forming the electrical interconnect layer includessimultaneously etching the electrical interconnect layer and the movablelayer using a first etchant.
 4. The method of claim 3, wherein the thinfilm is substantially resistant to etching by the first etchant.
 5. Themethod of claim 4, wherein the thin film is under the electricalinterconnect layer and the movable layer during simultaneous etching. 6.The method of claim 1, wherein the thin film comprises molybdenum. 7.The method of claim 1, wherein the sacrificial layer comprisesmolybdenum.
 8. The method of claim 1, further comprising performing arelease etch to remove the sacrificial layer to form an optical cavity.9. The method of claim 8, wherein the release etch also removes the thinfilm.
 10. The method of claim 8, further comprising protecting theelectrical interconnect and the thin film in the array region from therelease etch with an inorganic dielectric material.
 11. The method ofclaim 1, wherein the electrical interconnect layer comprises aluminum.12. The method of claim 1, wherein depositing the thin film is conductedbefore forming the movable layer.
 13. The method of claim 1, wherein thedevice is an interferometric modulator.
 14. A microelectromechanicalsystems device formed by the method of claim 1.